1. Field of the Invention
The present invention relates to shift control apparatus and method for a belt type continuously variable transmission which are capable of preventing a belt slip due to an abrupt shift.
2. Description of the Related Art
Recently, an attention has been paid to a continuously variable transmission which controls a shift ratio (or speed ratio) continuously in that a shift shock can be avoided or a superior fuel consumption efficiency can be achieved. Especially, many vehicles in which the continuously variable transmissions are equipped have been developed. The continuously variable transmission presently in a reduction into practice includes a belt type continuously variable transmission for a relatively low output engine and a toroidal type continuously variable transmission for a relatively high output engine. The belt type continuously variable transmission is constituted by a primary pulley linked to an output shaft of an engine, a secondary pulley linked to drive wheels, and a belt wound around both of the pulleys. A power generated by the engine is transmitted from the primary pulley to the secondary pulley via the belt and is transmitted to the drive wheels. At this time, a hydraulic (a secondary (pulley) pressure) set in accordance with a basic characteristic of the secondary pulley such as a transmission torque is acted upon the secondary pulley to provide a clamping force for the belt. A hydraulic (a primary pressure) acted upon the primary pulley is adjusted to control the shift ratio (the speed ratio) (or an effective radius ratio of each of the primary pulley and secondary pulley). Thus, a gear shift is carried out. In general, in such a gear shift control as described above, a target revolution speed of the primary pulley is set on the basis of a vehicle speed or an engine throttle opening angle and the primary pressure is controlled so that an actual revolution speed of the primary pulley is made coincident with the target revolution speed thereof. In other words, a controlled variable of the primary pressure is determined according to a deviation between the actual revolution speed of the primary pulley and the target revolution speed thereof. As the deviation between the actual revolution speed and the target revolution speed becomes large, the controlled variable of the primary pressure becomes large so that a shift speed (a variation speed) of the shift ratio becomes fast. Hence, if an abrupt depression on an accelerator pedal is carried out during an inertia driving with the accelerator pedal released, the target revolution speed is set to a high value in accordance with the throttle valve opening angle. Thus, the deviation between the actual revolution speed and the target revolution speed becomes abruptly large and the shift speed becomes extremely fast. At this time, although the fast shift speed makes a quick acceleration possible and this is desirable in terms of a shift performance, the extremely fast shift speed, in turn, causes the shift ratio to abruptly be varied so that a belt slip occurs. In the worst case, there is a possibility that the belt and the pulleys are damaged.
To prevent such a belt slip as described above, a Japanese Patent Application First Publication No. 2001-304389 published on Oct. 31, 2001 exemplifies a previously proposed shift control apparatus in which when the shift speed is equal to or faster than a preset predetermined value, an advance of the shift ratio control is suppressed so as to suppress the shift speed. Specifically, in the shift control apparatus in which a magnitude of the primary (pulley) pressure is corrected by means of a feedback control, when the shift speed is in excess of a threshold value placed in a vicinity to an upper limit shift speed below which the belt slip does not occur, a correction quantity (for example, an integration correction quantity, a proportional correction quantity, or a differential correction quantity) of a hydraulic control from among feedback correction quantities in a case where a time lag in a rise in actual revolution speed with respect to a variation in the primary pressure due to a mechanical response delay of the primary pulley is reset to zero. Thus, when the shift speed is equal to or below threshold value Vo, the correction of the feedback control can improve the shift performance. When the shift speed is in excess of threshold value Vo, the correction quantity of the feedback control is reduced and the rise in the shift speed can be suppressed.